The invention relates to the manufacture of pistons having a skirt on which are formed a plurality of bearing surfaces for transmitting lateral thrust from the piston to an associated cylinder or liner, each bearing surface being at a predetermined axial position on the skirt, being spaced outwardly of the skirt to a required radial dimension and extending around the skirt with a required circumferential dimension, hereinafter called "a piston of the kind referred to".
It has recently been discovered that improved lubrication and reduced friction between the piston and an associated cylinder or liner can be achieved by replacing the conventional generally cylindrical skirt with such individual bearing surfaces. The bearing surfaces are provided on both the thrust side and the counter-thrust side of the piston, which lie on opposite sides of the plane including the piston axis and the axis of a gudgeon pin bore of the piston. Two or more bearing surfaces are provided on each side and can be arranged in various configurations to give optimum performance. The bearing surfaces extend radially outwardly of the surrounding skirt portion by, in general, as little as 25 microns and are connected to the surrounding skirt by sloping ramps. In addition, the circumferential extent of each bearing surface is, in general, 15.degree. or 20.degree..
It will be appreciated that the small distance by which the bearing surfaces are spaced outwardly of the surrounding skirt and the limited circumferential extent of the bearing surfaces make the manufacture of such a piston more difficult than the manufacture of conventional pistons, which can be made by a conventional turning operation using conventional machine tools. In general, such machines tools do not, however, have sufficient flexibility to machine pistons of the kind referred to above at commercial rates of production.